1. Field of the Invention
The present invention relates to a liquid crystal projector for displaying an image using a liquid crystal display device as a spatial light modulation device.
2. Description of the Related Art
TN (Twisted Nematic) liquid crystal and ferroelectric liquid crystal are well known as liquid crystals used in display devices. Although TN liquid crystal display devices and ferroelectric liquid crystal display devices are both constructed such that the polarization properties of these devices can be used to control reflectivity (or transmittance), the operation of these two types of display devices is different.
While continuous control of reflectivity (or transmittance) is possible in a TN liquid crystal, reflectivity (or transmittance) can only be controlled discretely in two states (ON and OFF) in a ferroelectric liquid crystal. Thus, in the case of a ferroelectric liquid crystal, gray-scale display is realized by pulse width modulation (PWM). In other words, gray scale can be represented by controlling the time ratio of the ON and OFF states.
When the same screen is displayed on a liquid crystal display device over a long period of time, this screen will remain even after switching to another pattern, i.e., the phenomenon known as image burn-in or image sticking occurs. This phenomenon is thought to occur because, when an electric field of the same orientation acts on liquid crystal cells over a long period of time, ions of impurities that are contained in the liquid crystals are displaced toward the interface, and the electric field that is produced by these displaced ions of impurities then interferes with the movement of the liquid crystal.
In order to prevent image burn-in, control is effected in the liquid crystal display device such that the average electric field that is applied to the liquid crystal cells over a particular time period is zero. In other words, the display of one image must be followed by the application of the opposite electric field.
Since the states of polarized light are controlled by the absolute value of the electric field in a TN liquid crystal and are not related to polarity, the positive (normal gray-scale) image is displayed even if the polarity of the electric field is reversed. In the case of a ferroelectric liquid crystal, on the other hand, the states of polarized light are controlled by the polarity of the electric field, and reversing the polarity of the electric field causes a negative (inverted gray-scale) image to be displayed. The prior art is next explained with reference to the following patent documents:
Patent Document 1:
    Japanese Patent Laid-Open Publication No. 2002-244211Patent Document 2:    Japanese Patent Laid-Open Publication No. 2001-174775Patent Document 3:    Japanese Patent Laid-Open Publication No. H11-331879Patent Document 4:    Japanese Patent No. 2999952    (Japanese Patent Laid-Open Publication No. H9-138371)Patent Document 5:    Japanese Patent Laid-Open Publication No. H11-281931Patent Document 6:    Japanese Patent Laid-Open Publication No. H5-257110
Explanation first regards the first example of a liquid crystal projector of the prior art with reference to FIGS. 1, 2, and 3. FIG. 1 is a structural view showing the optical configuration of the first example of a liquid crystal projector of the prior art. Patent Document 1, for example, discloses an image projection device that employs polarized light conversion elements. The optical configuration of the first example of a liquid crystal projector of the prior art is described with reference to FIG. 1. The first example of the liquid crystal projector of the prior art is of a construction that employs one liquid crystal display device.
Red, green, and blue linearly polarized light beams are irradiated from red linearly polarized light source C11R, green linearly polarized light source C11G, and blue linearly polarized light source C11B, respectively. The paths of these linearly polarized light beams are unified by color synthesizing optics C13. The linearly polarized light beams that are transmitted by color synthesizing optics C13 are shaped by luminous flux shaping optics C12, and irradiated into liquid crystal display device C2.
Liquid crystal display device C2 spatially modulates the polarized state of the incident light and directs the modulated light into polarizing filter C3. Of the incident light, polarizing filter C3 transmits only light for which the polarization direction coincides with the axis of transmission. Light that has been transmitted by polarizing filter C3 passes through projection optics C4 and is projected as image light onto a screen (not shown in the figure).
FIG. 2 is a structural view showing the optical configuration of the linearly polarized light source that is shown in FIG. 1. Non-polarized light that is emitted by light-emitting element C111 is split into a P-polarized light beam and an S-polarized light beam by polarization beam splitter C1121. The optical path of this S-polarized light beam is deflected and made parallel to the P-polarized light beam by mirror C1122. The direction of polarization of the P-polarized light beam that is emitted from polarization beam splitter C1121 is rotated 90 degrees by half-wave plate C1123, its direction of polarization thereby being made the same as that of the S-polarized light beam that is emitted from polarization beam splitter C1121. In other words, the non-polarized light beam that is irradiated from light-emitting element C111 is converted to an S-polarized light beam by polarization conversion element array C112.
The control of the liquid crystal projector that is shown in FIG. 1 is next explained with reference to FIG. 3. FIG. 3 is a timing chart showing the states of control of the first example of the liquid crystal projector of the prior art that is shown in FIG. 1.
In FIG. 3, R-S polarized light indicates that the light is red and S-polarized light, G-S polarized light indicates that the light is green and S-polarized light, and B-S polarized light indicates that the light is blue and S-polarized light. In addition, R-Pos indicates a positive red image, R-Neg indicates a negative red image, G-Pos indicates a positive green image, G-Neg indicates a negative green image, B-Pos indicates a positive blue image, B-Neg indicates a negative blue image, and OFF indicates the absence of light.
Liquid crystal display device C2 is controlled so as to display images having the characteristics of R-Pos, G-Pos, B-Pos, R-Neg, G-Neg, and B-Neg in accordance with the video signals. Light beams are successively emitted from illumination system C1 in synchronization with these images, these light beams having the characteristics R-S polarized light, G-S polarized light, B-S polarized light, OFF, OFF, and OFF. Accordingly, polarizing filter C3 emits only positive image light that is displayed by liquid crystal display device C2 and does not emit light during the interval in which a negative image is displayed.
The above-described construction makes the average electric field that is applied to the liquid crystal zero in a ferroelectric liquid crystal display device and thus can prevent the burn-in phenomenon.
In the first example of a liquid crystal projector of the ferroelectric liquid crystal type such as is shown in FIG. 1, an image is not displayed for half of the interval as shown in FIG. 3, and this form therefore has the problem that the brightness of the projected image is sacrificed to prevent the burn-in phenomenon.
Explanation next regards the second example of a liquid crystal projector of the prior art with reference to FIGS. 4 and 5. FIG. 4 is a structural view showing the optical configuration of the second example of the liquid crystal projector of the prior art. This second example of a liquid crystal projector of the prior art is a construction that employs two liquid crystal display devices. A prior-art example that uses two liquid crystal display devices is disclosed in, for example, Patent Document 2.
The optical configuration of the liquid crystal projector of the prior art is first explained with reference to FIG. 4. White light that is emitted from white light source 111 is successively converted to red light, green light, and blue light by means of color switching means 112. The light is further transmitted or blocked by means of shutter 113.
Polarization beam splitter 102 is an optical element that allows rectilinear propagation of the P-polarized light beam but that deflects the optical path of the S-polarized light beam by 90 degrees. The non-polarized light that is emitted from illumination means 101 is resolved by polarization beam splitter 102 into a P-polarized light beam and an S-polarized light beam, and these light beams are directed to reflective liquid crystal display devices 103 and 104, respectively.
Reflective liquid crystal display device 103 receives the P-polarized light beam that has been transmitted by polarization beam splitter 102 and emits a P-polarized light beam and an S-polarized light beam at a proportion that is controlled in accordance with video signals. Of the light that is emitted by reflective liquid crystal display device 103, the P-polarized light beam is propagated directly through polarization beam splitter 102 without being directed toward projection optics 105 while the S-polarized light beam is deflected by polarization beam splitter 102 and directed toward projection optics 105.
Reflective liquid crystal display device 104 receives the S-polarized light beam that has been transmitted by polarization beam splitter 102 and emits a P-polarized light beam and an S-polarized light beam at a proportion that is controlled in accordance with video signals. Of the light that is emitted by reflective liquid crystal display device 104, the P-polarized light beam is propagated directly through polarization beam splitter 102 and is directed toward projection optics 105, and the S-polarized light beam is deflected by polarization beam splitter 102 and is not directed toward projection optics 105.
The light that is transmitted by polarization beam splitter 102 and reaches projection optics 105 is projected as image light upon a screen (not shown in the figure).
Referring now to FIGS. 4 and 5, the control of the liquid crystal projector that is shown in FIG. 4 is next explained. FIG. 5 is a timing chart showing the states of control when using a ferroelectric liquid crystal display device in the liquid crystal projector shown in FIG. 4.
In FIG. 5, R indicates red light, G indicates green light, B indicates blue light, and OFF indicates that there is no light. In addition, R-P polarized light indicates red P-polarized light, R-S polarized light indicates red S-polarized light, G-P polarized light indicates green P-polarized light, G-S polarized light indicates green S-polarized light, B-P polarized light indicates blue P-polarized light, and B-S polarized light indicates blue S-polarized light. Further, R-Pos indicates the display of a positive red image, R-Neg indicates the display of a negative red image, G-Pos indicates the display of a positive green image, G-Neg indicates the display of a negative green image, B-Pos indicates the display of a positive blue image, and B-Neg indicates the display of a negative blue image.
Light beams having the characteristics R (red light), OFF (no light), G (green light), OFF (no light), B (blue light), and OFF (no light) are successively emitted from illumination means 101. Illumination means 101 and reflective liquid crystal display devices 103 and 104 are controlled in synchronization with video signals.
During the intervals in which light is emitted from illumination means 101, reflective liquid crystal display devices 103 and 104 are controlled so as to display positive images in accordance with video signals. In addition, during the interval in which light is not emitted from illumination means 101, reflective liquid crystal display devices 103 and 104 are controlled so as to display negative images in accordance with video signals. Projection optics 105 therefore emits only the positive image light that is displayed by reflective liquid crystal display devices 103 and 104 and does not emit light during the intervals in which negative images are displayed.
The above-described construction makes the average electric field that is applied to the liquid crystal zero in a ferroelectric liquid crystal display device and thus can prevent the uneven distribution of ions and prevent the burn-in phenomenon. Thus, the second example of a liquid crystal projector of ferroelectric liquid crystal of the prior art as shown in FIG. 4 also has the problem that, as with the first example, an image is not displayed during half of the intervals, as shown in FIG. 5, and the brightness of the projected image is therefore sacrificed to prevent the burn-in phenomenon.
The display of a stereoscopic image is one application of a liquid crystal projector. Several methods are used for stereoscopic display.
A stereoscopic image projector and an apparatus for stereoscopic viewing of an image, this projector and apparatus being the invention of Patent Document 3, disclose a method of stereoscopic display by projecting a left-eye image and a right-eye image, which are linearly polarized light beams directed in the same direction, and glasses that include a liquid crystal shutter that acts to alternately block the right-eye line of sight and the left-eye line of sight.
In addition, a stereoscopic image display device that uses polarization glasses, this device being the invention of Patent Document 4, discloses a method for stereoscopic display that is realized by alternately projecting a left-eye image and a right-eye image, these images being linearly polarized light that is polarized in different directions, and by restricting the optical paths by means of polarization glasses.
In addition, the projector that is the invention described in Patent Document 5 and the projector-type liquid crystal display device that is the invention described in Patent Document 6 both disclose a method for stereoscopic display that is realized by limiting optical paths by means of polarization glasses and by projecting a left-eye image in combination with a right-eye image, these images being linearly polarized light that is polarized in different directions.
In the liquid crystal projectors that are applied in these stereoscopic image displays, the display device may employ a TN liquid crystal or a ferroelectric liquid crystal.